Latex on wet screed

[bolst3r]

My builders have laid a new screed but the height seems a little low for floor tiles in one place so is it best for them to add more screed in the area where it's low or add a latex leveler? I need about 12-15mm taken up across a 5x2meter area. Is it ok to lay screed on screed and what if the first layer isn't fully dry, does this matter?

Eventually the floor is going to be tiled and ditra matting will be used in case it's not fully dry and because there are several floors being combined.

Thanks for any help